Leupeptin, a modified tripeptide, can inhibit kallikrein as well as other proteases. The purpose of the current investigation was to evaluate the potency of leupeptin as a kallikrein inhibitor. The TAME esterase assay was used to measure kallikrein activity in the presence and absence of leupeptin. Dixon plots were made and Ki calculated. The Ki is on the order of 10 to the minus 5th power M which is 2 to 4 orders of magnitude less potent than other kallikrein inhibitors. This makes leupeptin much less attractive for current studies.